Parallel membranous layer type fluid diffusion cell

ABSTRACT

A fluid diffusion cell having a diffusion membrane folded to define a first set and a second set of oppositely opening pockets. Each pocket in both sets contains a support member, and a pair of manifolds are provided to provide a fluid inlet and outlet at opposite ends of each of said sets of pockets to permit fluid to pass from each inlet into one end of each of the pockets of one set, to pass through each of the pockets and the outlet.

United States Patent Inventor Ronald James Leonard Elk Grove Village,111. Appl. No. 6,015 Filed Jan. 26, 1970 Division of Ser. No. 712,066,Mar. 11, 1968, Pat. No. 3,560,340. Patented Oct. 12, 1197 1 AssigneeBaxter Laboratories, llnc.

Morton Grove, 1111.

PARALLEL MEMBRANOUS LAYER 'llYlPE lFLUllD DIFFUSION CELL 14 Claims, 9Drawing Figs.

US. Cl 210/321, 23/2585 Int. Cl A6lm l/03, B01d 13/00 Field of Search195/18; 210/22, 23; 23/2585 References Cited UNITED STATES PATENTS3,396,849 8/1968 Lande et a1. 23/2585 X 3,413,095 11/1968 Bramson23/2585 3,503,850 3/1970 Dibelius 23/2585 X 3,318,747 5/1967 Presser eta1 156/204 OTHER REFERENCES William G. Esmond et al.; ProfoundHypothermia With Simplified Equipment: A Disposable Stainless Steel HeatExchanger of High Efficiency Journal of Thoracic and CardiovascularSurgery; Vol. 42, No. 5, Nov. 1961; pp. 563- 574 (Copy in 23/2585)Primary Examiner-Morris O. Wolk Assistant Examiner-Barry S. RichmanAttorneys walter C. Kehm, Richard J. Reilly and W.

Garrettson Ellis ABSTRACT: A fluid diffusion cell having a diffusionmembrane folded to define a first set and a second set of oppositelyopening pockets. Each pocket in both sets contains a support member, anda pair of manifolds are provided to provide a fluid inlet and outlet atopposite ends of each of said sets of pockets to permit fluid to passfrom each inlet into one end of each of the pockets of one set, to passthrough each of the pockets and the outlet.

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lfAllltAlLlLlElL MllilltllhhANUlUS LAifhllt TYPE ll' 'lUUlllllrlllllllilflldlltllhl lClElLlL This application is a division ofapplication, Ser. No. 712,066, filed ll/lar. ll, 1968.

The present invention relates to fluid diffusion devices in whichmaterial transfer occurs across a selectively permeable membranedisposed between first and second fluids. Particularly the inventionrelates to a diffusion cell for oxygenating or dialyzing blood by fluidpassage through a laminous structure, such as that which is provided bya continuous folded or pleated selectively permeable membrane.

As heretofore known device of the class with which the present inventionis concerned dialyzes blood through an unsupported flexible foldedmembrane which provides a plurality of parallel flow paths for bloodalong one face of the membrane and a plurality of countercurrent flowpaths for dialyzing fluid along the other face of the membrane. Thedevice is adapted for blood and dialyzing fluid which are movedintermittently and reciprocally by pumping them alternately andcountercurrently. in consequence thereof the poclrets formed along oneface of the membrane fill while the poclrets formed along the other faceof the membrane empty. The device is itself an effective pump andrequires valving for fluid flow in a manner intended. As a result of theconstantly alternating condition of the membrane thereto incidentvariation in blood treatment results. Moreover, because of the requiredlack of rigidity in the membrane, the likelihood of generating shortcircuits of the regular paths through the device give rise to aninherent hazard of inadequate blood treatment.

An object of the present invention is the provision of an improved fluiddiffusion device of the laminated membrane type for treating blood.

Another object of the invention is to ensure uniform fluid flow througha blood oxygenator or blood dialyzer of the designated class.

An additional object of the invention is to improve the oxygen andcarbon dioxide exchange characteristics in a fluid diffusion cell.

To effect the foregoing objects, a fluid diffusion cell with a laminatedselectively permeable membrane structure has a plurality of oppositelyopening pockets which define a pair of sets of parallel flow paths alongcorresponding surfaces of the laminae of said structure. A fluid inletmember and a fluid outlet member are associated with each set ofparallel paths for introducing therein and removing therefrom blood andother fluid separated each from the other by said laminae. A rigidsupport member is disposed in each pocket for filming thereintherethrough flowing fluid, and maximizing fluid contact with saidlaminae.

How to further accomplish the foregoing and other objects, features andadvantages of the invention will become more apparent upon considerationof the following description and appended claims, when considered inconjunction with the accompanying drawings wherein the same referencecharacter or numeral refers to like or corresponding parts throughoutthe several views.

On the drawings:

lFlG. l is a perspective view of a diffusion cell embodying the presentinvention.

FIG. 2 is a view according to the section line 2-2 of FIG. ll.

FIG. 3 is a view according to section line 3-3 of FlG. 2, spacer-screenshaving been omitted for the purpose of illustration.

FllG. i is an enlarged view of a section according to the line d-d ofH6. 2.

FIG. 5 is a perspective view of a folded membrane comprising thediffusion cell.

FIG. ti is a perspective view of one type of spacer-screen comprisingthe diffusion cell.

F lG. 7 is a perspective view of another type of spacer-screencomprising said diffusion cell.

FIG. fl is a perspective of a manifold for supplying fluid in saiddiffusion cell viewed from one aspect.

FIG. 9 is a perspective of said manifold viewed from another aspect.

Referring now more particularly to the drawings, a fluid diffusion cellgenerally designated ll0 (FllG. l) is illustrated herein as a device fortreating a body fluid such as blood. For the purpose of orientation,cell llll may be considered as hav ing a top end portion H, a bottom endportion lid, a front face lit, a thereto parallel bacl: face (not shown)and opposed lateral sides, being a blood side lb and a side 26 foranother fluid.

Cell 10 comprises a laminated membranous structure having parallellaminae which may be formed by the courses of a preferably flexible andselectively permeable membrane 22 (MG. 5). The latter is folded orpleated to provide a first set 2d of pockets ZdA, 243, 241C, Z illD andME which open outwardly toward the blood side lb, and a second set 28 ofpockets NM, 28B, 28C and 28D which open outwardly toward the other side20. The pockets of each set define parallel flow paths alongcorresponding surfaces of the laminae.

Membrane 22 is fabricated preferably of a biologically inert base, suchas a mesh of fiber glass or Dacron about 0.005-inch thiclr. Over thelatter there is applied a very thin cover of an elastomer, preferablysilicone rubber, sufficient only to fill the spaces defined by the mesh;In a preferred process for the cell, membrane 22 is pleated intooppositely directed membranous courses or layers to form uniformlydimensioned pockets about a plurality of rectangular parallel formingplates (not shown). The latter are supported in horizontal array forseparating adjacent courses or pleats by about 0.040 inch. Thereafter,the top edge portions 32 of the pleats forming top end portion 112 ofcell and the bottom edge portions 34 of the pleats forming bottom endportion M of said cell are sealed together in fixed and spaced-apartrelationship with a sealing material 30. In consequence, sealed top andbottom ends of said pockets are formed. An MTV silicone rubber providesa suitable sealing material and imparts reasonable rigidity to top andbottom end portions l2 and lid of the cell. Rubber sealing is preferredto metal ties and preformed sealing components because its use minimizesproduction costs.

The structure thus formed enables separation of blood and another fluidfor flow in respective parallel flow paths along corresponding faces ofthe membranous laminae by which such fluids are completely separated. inconsequence of the foregoing, the flow of blood can be confined to thepockets of one set {herein set M) and the flow of the other fluid can beconfined to the pockets of the other set (herein set 2%). When cellllti) is an oxygenator, as shown herein, the other fluid will be oxygen.However, the device is not limited to oxygenators and may be employedwith equal effect as a dialyzer. In that event the other fluid would bea dialyzing liquid.

After sealing the top and bottom end portions l2 and M, the rectangularforming plates (not seen) are withdrawn from the pockets. Thereafter, asupport member as of a first type, and herein comprising a spacer-screenwhich is substantially the same height and width as a pocket and ofrectangular profile, is inserted into each of the oxygen pockets (thepockets of set 28). A support member of a second type, and hereincomprising a spacer-screen 3b which is of a rectangular profile andabout the seam width as a pocket but shown as being shorter than thesame, is inserted into each of the blood pockets (the pockets of set24).

Spacer-screen 36 has a top section dill, a bottom section d2, a medialsection as, opposed lateral side portions M and 46, and a top surface48. Spacer-screen 3% has a top portion 50, a bottom portion 52, opposedlateral side portions 54 and 56, and a top surface 5%. The length andwidth of each last spacerscreen is substantially the same as thecorresponding dimensions of a medial section 66. Each pocket has thesame height and width as the others; and as illustrated in FIG. 2, whichshows the interior of a blood pocket, each pocket is characterized by anupper end portion 60, a thereto opposed lower end portion 62 and amedial portion rid. While each spacerscreen 36 extends substantially theentire length (between top and bottom) of the pocket in which it isdisposed, each spacerscreen 338 is proportioned for disposition only inthe medial portion 6 3 of its pocket and extends neither into theassociated top portion 60 nor into bottom portion 62. In the illustratedembodiment, although the depth of each pocket (distance between adjacentlaminae) preferably is 0.040 inch, immediately after setting asaforesaid, the depth of each spacer-screen 38 is 0.020 inch. On theother hand, medial section 66 of each spacer-screen 36 is 0.060 inchwhile its top and bottom end sections 40 and 42, which are substantiallyof the same length are 0.020 inch. By reason of the foregoingspacerscreen 36 can be considered as having an enlarged medial sectionor reduced end sections.

Each spacer-screen 36 may be a mesh of fiber glass fabrication coveredwith vinyl of a thickness insufficient to mask its screen character.Each spacer-screen 38, on the other hand, preferably is of plasticfabrication, fashioned from low density polyethylene by extrusion.

In lieu of spacer-screens, a pair of rectangular rigidifying elements 72and 74 (FIG. 4) may be inserted or disposed in pockets 24A and 24E.These members preferably are about 0.020 inch thick and of an area forfilling their pockets. They are of any suitable construction, such asfiber glass reinforced with phenolic plastic.

A pair of preferably like structured manifolds, only one of which isshown in FIGS. 8 and 9, comprise a blood manifold 75 and an oxygenmanifold 76. They respectively are mounted on cell sides 18 and 20. Eachmanifold has a pair of spacedapart, top and bottomlike proportioned,inlet and outlet, end receptacles or wells 80 and 82 which are connectedtogether by an integral medial section 84. Each inlet well 80, togetherwith an inlet duct 88 comprises inlet means whereas each outlet 82together with an outlet duct 90 comprises outlet means. Moreover, eachmanifold has, (1) a pair of parallel front and rear walls 78, theopposite top and bottom portions of which define the front and rearfaces of said wells, and (2) a pair of parallel top and bottom walls 79which define the top and bottom, respectively, of wells 80 and 82.Furthermore, medial section 84 of each manifold has an inwardly ofisetflat surface 81 which together with walls 78, 78 and walls 79, 79 definea chamber of trough 83 for reception of an associated side of the foldedmembrane with therein disposed spacer-screens. The parts areproportioned so that (l) the walls 78, 78 of each manifold, on itsrespective side, engage front and rear surfaces of the folded membrane,(2) walls 79, 79 of each manifold engage the seals 30 at the top andbottom of the membrane, and (3) each flat surface 81 engages against themedial section of its associated side of the pockets, when the cell isassembled. In such condition all adjoining surfaces are securedtogether, preferably with an RTV material for sealing silicone rubberfrom which the manifolds preferably are fabricated.

As a result of the foregoing, the inner surface 81 of manifolds 75 issecured along side 18 to the folds defining pockets 28A, 28B, 28C and28D and closes the medial sections of the openings of the pockets of set24 toward side 18 while leaving the upper and lower end portions 60 and62 of said last pockets arranged in immediate fluid communication withwells 80 and 82 of said last manifold. Similarly, inner surface 81 ofmanifold 76 is secured along side 20 to the folds of pockets 24A, 24B,24C, 24D and 24E, and closes the medial sections of the openings of thepockets of set 28 toward side 20 while leaving the upper and lowerportions 60 and 62 of said last pockets arranged in immediate fluidcommunication with wells 80 and 82 of the last-mentioned manifold.

The illustrated cell is adapted for supplemental and support oxygenationof blood. To that end, the blood inlet and outlet means are adapted forconnection to the circulatory system of an individual to be serviced,respectively, for receiving and for returning blood from and to suchindividual without the assistance of an auxiliary pump. The parts ineach blood pocket are arranged for moving blood according to directionalarrows (FIG. 2) from blood inlet well 80, at which the upstream end oropening of each blood pocket immediately is disposed, along the upperportion 60 of such pocket and therefrom downstream through acorresponding lower portion 62 for return from blood outlet well 82 atwhich the downstream end or opening of each pocket immediately isdisposed. The oxygen manifold 76 and oxygen pockets (of set 28) may besimilarly arranged for flow in its pockets. However, in a dialyzer flowof blood and the other fluid preferably will be in countercurrent paths.

As illustrated in FIG. 4, the opposite surfaces of each spacer-screen isengaged by an adjoining lamina. This arrangement results in thedevelopment of a pair of enlarged opposed channels having end openingsand defined in each blood pocket at its upper and lower end portions 60and 62. These openings are disposed for immediate communication withblood inlet and outlet means of manifold 75; and the channels herein areabout 0.060 inch in depth and extend across the top and bottom of thecell from side to side adjacent seals 30. They provide resistance of nopractical significance whereby blood entering the cell rapidly flowsacross the tops of all of the blood pockets and the flow of bloodleaving is not needlessly impeded. The depth of the medial portion 64 ofeach blood pocket is limited to 0.020 inch, the depth of its bloodspacer-screen, against which a pair of laminae are held by the medialsections 66 of adjacent spacer-screens 36. This causes the blood to bespread in a very thin film, as a consequence of which, oxygen exchange,which is a function of blood film thickness, is exceedingly rapid.

The cell is constructed so that almost maximum oxygenation can occurwith laminae of minimum total surface area in sections 64 of the bloodpockets and while maintaining desired output of blood introduced underblood pressure of an individual being treated. Such surface area,however, is inadequate for liberation of all the CO from normal venousblood. Additional surface area required for adequate CO elimination fromthe blood is provided by the sections of the laminae which define endportions 60 and 62 of the blood pockets. An efficient cell has anexchange surface area in the channel portions 60 and 62 of the bloodpockets which is not less than about 15 percent of the exchange surfacearea of the blood-filming portions of the cell.

In the oxygen pockets adjacent the blood channels a plurality of canals92 (FIG. 4), which extend from side to side parallel to the bloodchannels, develop at the front and back of screen sections 40 and 42because of the depth of said screen sections and the spacing of thelamina whose top and bottom edges are anchored in fixed relationship.Canals 92 form easy entries into and exits from the oxygen pockets andare disposed at respective oxygen inlet and outlet wells 80 and 82.

In addition to providing great efficiency in oxygenating and dialyzingblood, the invention enables construction of a cell at substantiallyless cost than related heretofore known devices of the same type. Thislast feature is highly significant in facilitating widespread enjoymentof the invention.

As many substitutions or changes could be made in the above-describedconstruction and process and as many apparently widely differentembodiments of the invention within the scope of the claims could beconstructed without departing from the scope and spirit thereof, it isintended that all matter contained in the accompanying specificationshall be interpreted as being illustrative and not in a limiting sense.

What is claimed is:

l. A fluid difiusion cell, including a selectively permeable diffusionmembrane, folded to form laminae defining alternately positioned firstand second sets of pockets, the first set of pockets opening at theopposite side of the membrane to the second set of pockets, theimprovement comprising first fluid inlet means and first fluid outletmeans in communication with the first set of pockets and located at themouths thereof, and second fluid inlet means and second fluid outletmeans in communication with the second set of pockets and located at themouths thereof, each inlet and outlet means having an enlarged externalchamber, open to the mouths of each pocket of a set, to provide flowpaths to each pocket of approximately equal length, a medial memberbetween each inlet and outlet means pressing against the mouths of saidpockets to force fluid to flow into and through said pockets whilepassing from each inlet to each outlet means, the first and second setsof pockets defining separate parallel flow paths for respective fluids,and a support member disposed in each of said pockets for holdingadjacent laminae apart from each other and for providing flow paths ofpredetermined thickness, the length of said flow paths in the pocketsbeing greater than the dimension of said fluid diffusion cell which isperpendicular to said laminae.

2. The diffusion cell of claim 1 in which said support members in thepockets comprise screens made of a mesh of crossing strands.

3. The fluid diffusion cell of claim l in which said diffusion membranecomprises a mesh over which is applied a thin cover of silicone rubbersufficient to fill the spaces of said mesh.

4. The diffusion cell of claim 3 in which said support membars in thepockets comprise screens made of a mesh of crossing strands.

5. The fluid diffusion cell of claim 4 in which end portions of thefluid diffusion cell are sealed with an elastomeric sealant.

6. The fluid diffusion cell of claim 5 in which one of said sets ofpockets receives blood while the other of said sets of pockets receivesoxygen.

7. A fluid diffusion cell including a selectively permeable diffusionmembrane, folded to form laminae defining altemately positioned firstand second sets of pockets, the first set of pockets opening at theopposite side of the membrane to the second set of pockets, manifoldmeans in communication with the first and second sets of pockets andlocated at the mouths thereof to feed and withdraw fluid in a flow paththrough said pockets, and support members disposed in each of saidpockets for holding adjacent laminae apart from each other and forproviding flow paths of predetermined thickness, the support members inthe pockets of said first set extending the entire length of saidpockets, while the support members in the pockets of the second setterminate short of the ends of said pockets, whereby unobstructedchannels are defined in the ends of the pockets of the second set.

8. A combination according to claim 7 in which each support member inthe pockets of one set is of substantially uniform thickness and hasopposite faces in contact with adjoining laminae, and each supportmember in the pockets of said other set has an enlarged medial sectionand opposed smaller end sections, said medial section and smaller endsections having opposite faces in contact with adjoining laminae forforming said channels.

9. A blood oxygenator including a selectively permeable diffusionmembrane, folded to form laminae defining alternately positioned firstand second sets of pockets, the first set of pockets opening at theopposite side of the membrane to the second set of pockets, manifoldmeans in communication with the mouths of each of the first and secondsets of pockets to feed and withdraw blood and oxygen in separate flowpaths, each through a separate set of pockets, and a support memberdisposed in each of said pockets for holding adjacent laminae apart fromeach other and for providing flow paths of predetermined thickness, inwhich portions of said support members adjacent the ends thereof are ofreduced aggregate thickness, to provide a portion of said blood flowpath of increased thickness in contact with said membrane and oxygenflow path, whereby blood flowing through said blood flow path portion ofincreased thickness is conditioned for relatively increased carbondioxide diffusion through said membrane, while along the portions ofsaid support members remote from the ends thereof, blood is conditionedfor relatively increased oxygen diffusion through said membrane.

110. The blood oxygenator of claim 9 in which said diffusion membranecomprises a mesh over which is applied a thin cover of silicone rubbersufficient to fill the spaces of said mesh.

1111. The blood oxygenator of claim 9 in which said support memberscomprise screens made of a mesh of crossing rand H2. The bloodoxygenator of claim 9 in which the exchange surface area of said flowpath portions of increased thickness is not less than 15 percent of theexchange surface area of the remaining flow path portions within saidpockets.

113. A fluid diffusion cell, including a selectively permeable diffusionmembrane, folded to form laminae defining alternately positioned firstand second sets of pockets, the first set of pockets opening at theopposite side of the membrane to the second set of pockets, theimprovement comprising first fluid inlet means and first fluid outletmeans in communication with the first set of pockets and located at themouths thereof, and second fluid inlet means and second fluid outletmeans in communication with the second set of pockets and located at themouth thereof, each inlet and outlet means hav ing an enlarged externalchamber, open to the mouths of each pocket of a set, to provide flowpaths to each pocket of approximately equal length, a medial memberbetween each inlet and outlet means pressing against the mouths of saidpockets to force fluid to flow into and through said pockets whilepassing from each inlet to each outlet means, the first and second setsof pockets defining separate parallel flow paths for respective fluids,and a support member disposed in each of said pockets for holdingadjacent laminae apart from each other and for providing flow paths ofpredetermined thickness, the length of said flow paths in the pocketsbeing greater than the dimension of said fluid diffusion cell which isperpendicular to said laminae, said selectively permeable diffusionmembrane comprising a meshlike support element of biologically inertmaterial, in which the interstitial spaces defined by the meshlikesupport element are filled with a thin film of selectively permeableelastomer material.

141. The fluid diffusion cell of claim 113 in which said support membersin the pockets comprise screens made of a mesh of crossing strands.

1. A fluid diffusion cell, including a selectively permeable diffusionmembrane, folded to form laminae defining alternately positioned firstand second sets of pockets, the first set of pockets opening at theopposite side of the membrane to the second set of pockets, theimprovement comprising first fluid inlet means and first fluid outletmeans in communication with the first set of pockets and located at themouths thereof, and second fluid inlet means and second fluid outletmeans in communication with the second set of pockets and located at themouths thereof, each inlet and outlet means having an enlarged externalchamber, open to the mouths of each pocket of a set, to provide flowpaths to each pocket of approximately equal length, a medial memberbetween each inlet and outlet means pressing against the mouths of saidpockets to force fluid to flow into and through said pockets whilepassing from each inlet to each outlet means, the first and second setsof pockets defining separate parallel flow paths for respective fluids,and a support member disposed in each of said pockets for holdingadjacent laminae apart from each other and for providing flow paths ofpredetermined thickness, the length of said flow paths in the pocketsbeing greater than the dimension of said fluid diffusion cell which isperpendicular to said laminae.
 2. The diffusion cell of claim 1 in whichsaid support members in the pockets comprise screens made of a mesh ofcrossing strands.
 3. The fluid diffusion cell of claim 1 in which saiddiffusion membrane comprises a mesh over which is applied a thin coverof silicone rubber sufficient to fill the spaces of said mesh.
 4. Thediffusion cell of claim 3 in which said support members in the pocketscomprise screens made of a mesh of crossing strands.
 5. The fluiddiffusion cell of claim 4 in which end portions of the fluid diffusioncell are sealed with an elastomeric sealant.
 6. The fluid diffusion cellof claim 5 in which one of said sets of pockets receives blood while theother of said sets of pockets receives oxygen.
 7. A fluid diffusion cellincluding a selectively permeable diffusion membrane, folded to formlaminae defining alternately positioned first and second sets ofpockets, the first set of pockets opening at the opposite side of themembrane to the second set of pockets, manifold means in communicationwith the first and second sets of pockets and located at the mouthstHereof to feed and withdraw fluid in a flow path through said pockets,and support members disposed in each of said pockets for holdingadjacent laminae apart from each other and for providing flow paths ofpredetermined thickness, the support members in the pockets of saidfirst set extending the entire length of said pockets, while the supportmembers in the pockets of the second set terminate short of the ends ofsaid pockets, whereby unobstructed channels are defined in the ends ofthe pockets of the second set.
 8. A combination according to claim 7 inwhich each support member in the pockets of one set is of substantiallyuniform thickness and has opposite faces in contact with adjoininglaminae, and each support member in the pockets of said other set has anenlarged medial section and opposed smaller end sections, said medialsection and smaller end sections having opposite faces in contact withadjoining laminae for forming said channels.
 9. A blood oxygenatorincluding a selectively permeable diffusion membrane, folded to formlaminae defining alternately positioned first and second sets ofpockets, the first set of pockets opening at the opposite side of themembrane to the second set of pockets, manifold means in communicationwith the mouths of each of the first and second sets of pockets to feedand withdraw blood and oxygen in separate flow paths, each through aseparate set of pockets, and a support member disposed in each of saidpockets for holding adjacent laminae apart from each other and forproviding flow paths of predetermined thickness, in which portions ofsaid support members adjacent the ends thereof are of reduced aggregatethickness, to provide a portion of said blood flow path of increasedthickness in contact with said membrane and oxygen flow path, wherebyblood flowing through said blood flow path portion of increasedthickness is conditioned for relatively increased carbon dioxidediffusion through said membrane, while along the portions of saidsupport members remote from the ends thereof, blood is conditioned forrelatively increased oxygen diffusion through said membrane.
 10. Theblood oxygenator of claim 9 in which said diffusion membrane comprises amesh over which is applied a thin cover of silicone rubber sufficient tofill the spaces of said mesh.
 11. The blood oxygenator of claim 9 inwhich said support members comprise screens made of a mesh of crossingstrands.
 12. The blood oxygenator of claim 9 in which the exchangesurface area of said flow path portions of increased thickness is notless than 15 percent of the exchange surface area of the remaining flowpath portions within said pockets.
 13. A fluid diffusion cell, includinga selectively permeable diffusion membrane, folded to form laminaedefining alternately positioned first and second sets of pockets, thefirst set of pockets opening at the opposite side of the membrane to thesecond set of pockets, the improvement comprising first fluid inletmeans and first fluid outlet means in communication with the first setof pockets and located at the mouths thereof, and second fluid inletmeans and second fluid outlet means in communication with the second setof pockets and located at the mouth thereof, each inlet and outlet meanshaving an enlarged external chamber, open to the mouths of each pocketof a set, to provide flow paths to each pocket of approximately equallength, a medial member between each inlet and outlet means pressingagainst the mouths of said pockets to force fluid to flow into andthrough said pockets while passing from each inlet to each outlet means,the first and second sets of pockets defining separate parallel flowpaths for respective fluids, and a support member disposed in each ofsaid pockets for holding adjacent laminae apart from each other and forproviding flow paths of predetermined thickness, the length of said flowpaths in the pockets being greater than the dimension of said fluiddiffusion cell which is perpendicular To said laminae, said selectivelypermeable diffusion membrane comprising a meshlike support element ofbiologically inert material, in which the interstitial spaces defined bythe meshlike support element are filled with a thin film of selectivelypermeable elastomer material.
 14. The fluid diffusion cell of claim 13in which said support members in the pockets comprise screens made of amesh of crossing strands.